Janet Liversidge

Constitutive Retinal CD200 Expression Regulates Resident Microglia and Activation State of Inflammatory Cells during Experimental Autoimmune Uveoretinitis

Recent evidence supports the notion that tissue OX2 (CD200) constitutively provides down-regulatory signals to myeloid-lineage cells via CD200-receptor (CD200R). Thus, mice lacking CD200 (CD200(-/-)) show increased susceptibility to and accelerated onset of tissue-specific autoimmunity. In the retina there is extensive expression of CD200 on neurons and retinal vascular endothelium. We show here that retinal microglia in CD200(-/-) mice display normal morphology, but unlike microglia from wild-type CD200(+/+) mice are present in increased numbers and most significantly, express inducible nitric oxide synthase (NOS2), a macrophage activation marker. Onset and severity of uveitogenic peptide (1-20) of interphotoreceptor retinoid-binding protein-induced experimental autoimmune uveoretinitis is accelerated in CD200(-/-) mice and although tissue destruction appears no greater than seen in CD200(+/+) mice, there is continued increased ganglion and photoreceptor cell apoptosis. Myeloid cell infiltrate was increased in CD200(-/-) mice during experimental autoimmune uveoretinitis, although NOS2 expression was not heightened. The results indicate that the CD200:CD200R axis regulates retinal microglial activation. In CD200(-/-) mice the release of suppression of tonic macrophage activation, supported by increased NOS2 expression in the CD200(-/-) steady state accelerates disease onset but without any demonstration of increased target organ/tissue destruction.

The role of tumour necrosis factor (TNF-α) in experimental autoimmune uveoretinitis (EAU)

The pleiotropic cytokine tumour necrosis factor-alpha (TNF-alpha) is released from cells that include macrophages and T-cells during inflammatory responses, orchestrating the initiation of further leucocytic infiltration via adhesion molecule upregulation, dendritic cell maturation and survival, macrophage activation and driving Th1 T-cells responses within tissues. Exposure to TNF also plays a role in maintaining tissue homeostasis, particularly relating to resident cell responses of both microglia and retinal pigment epithelium. Depending on the balance between duration and dose of TNF exposure, an environment where full expression of inflammatory and autoimmune responses within tissues may occur. In experimental autoimmune uveoretinitis (EAU), increased tissue concentrations of TNF facilitate the on-going T-cell effector responses and macrophage activation. These are responsible for targeted and bystander tissue damage and can be suppressed by anti-TNF therapies, in particular, those directed at the p55 TNF receptor. The ability to suppress disease experimentally has led to the successful translation of anti-TNF therapy for treatment of uveitis in cohort studies and phase I/II trials where, additionally, altered peripheral blood CD4(+) T-cell profiles can be demonstrated following each treatment.

Comparison of clinical and experimental uveitis

Uveitis is a term which encompasses many clinical syndromes which would appear to be discrete entities. Both clinically and experimentally, the separation of uveitis affecting only the anterior segment from that affecting the posterior segment has a sound pathogenetic basis. However, clear distinctions among the various forms of endogenous posterior uveitis are more difficult to maintain in the light of evidence from experimental models of autoimmune uveitis (EAU). EAU can be induced by a variety of retinal antigens and each antigen has been shown to induce somewhat different forms of EAU, depending on such factors as dose of antigen, species and strains of animal model, and the type(s) of adjuvant used. However, within each model a similar spectrum of uveoretinal responses can be induced by each antigen suggesting that the pathogenetic mechanisms are probably similar also. In addition, if these models are analogous to human disease, then each clinical entity within this apparently heterogeneous group of clinical posterior uveitis syndromes may represent one aspect of a general organ-specific uveoretinal response to autoantigens.

Control of myeloid activity during retinal inflammation

Combating myeloid cell‐mediated destruction of the retina during inflammation or neurodegeneration is dependent on the integrity of homeostatic mechanisms within the tissue that may suppress T cell activation and their subsequent cytokine responses, modulate infiltrating macrophage activation, and facilitate healthy tissue repair. Success is dependent on response of the resident myeloid‐cell populations [microglia (MG)] to activation signals, commonly cytokines, and the control of infiltrating macrophage activation during inflammation, both of which appear highly programmed in normal and inflamed retina. The evidence that tissue CD200 constitutively provides down‐regulatory signals to myeloid‐derived cells via cognate CD200‐CD200 receptor (R) interaction supports inherent tissue control of myeloid cell activation. In the retina, there is extensive neuronal and endothelial expression of CD200. Retinal MG in CD200 knockout mice display normal morphology but unlike the wild‐type mice, are present in increased numbers and express nitric oxide synthase 2, a macrophage activation marker, inferring that loss of CD200 or absent CD200R ligation results in “classical” activation of myeloid cells. Thus, when mice lack CD200, they show increased susceptibility to and accelerated onset of tissue‐specific autoimmunity.

Mechanisms of leukocyte migration across the blood-retina barrier

Immune-mediated inflammation in the retina is regulated by a combination of anatomical, physiological and immuno-regulatory mechanisms, referred to as the blood–retina barrier (BRB). The BRB is thought to be part of the specialised ocular microenvironment that confers protection or “immune privilege” by deviating or suppressing destructive inflammation. The barrier between the blood circulation and the retina is maintained at two separate anatomical sites. These are the endothelial cells of the inner retinal vasculature and the retinal pigment epithelial cells on Bruch’s membrane between the fenestrated choroidal vessels and the outer retina. The structure and regulation of the tight junctions forming the physical barrier are described. For leukocyte migration across the BRB to occur, changes are needed in both the leukocytes themselves and the cells forming the barrier. We review how the blood–retina barrier is compromised in various inflammatory diseases and discuss the mechanisms controlling leukocyte subset migration into the retina in uveoretinitis in more detail. In particular, we examine the relative roles of selectins and integrins in leukocyte interactions with the vascular endothelium and the pivotal role of chemokines in selective recruitment of leukocyte subsets, triggering adhesion, diapedesis and migration of inflammatory cells into the retinal tissue.

Nasal administration of retinal antigens suppresses the inflammatory response in experimental allergic uveoretinitis : a preliminary report of intranasal induction of tolerance with retinal antigens

Current immunotherapy of posterior uveitis is non-specific and limited by drug toxicity and unpredictable relapses on therapy. Alternative modes of therapy being investigated using the rat model of experimental autoimmune uveoretinitis (EAU) have included the induction of tolerance with oral administration of milligram quantities of retinal antigens. In this preliminary report we demonstrate that tolerance to retinal antigens can be induced via the upper respiratory tract with microgram doses of antigen, preventing subsequent induction of EAU.

Immunomodulation of experimental autoimmune uveoretinitis: A model of tolerance induction with retinal antigens

Experimental autoimmune uveoretinitis (EAU) is a CD4+ T-lymphocyte mediated inflammation of the uveal tract and retina. As a model of human posterior uveitis it permits further understanding of the underlying immunopathogenesis of uveitis. It also allows for preclinical trials of immunosuppressive therapies and in vivo assessment of alternative strategies for immunointervention. This review highlights possible immunostrategic modalities which prevent the initiation or perpetuation of the immune response, and in particular reports on the novel effect of intranasal induction of tolerance with retinal antigens, prior to immunisation with retinal antigens. The mechanisms and potential application of this ‘natural’ method of immunosuppression in the treatment of autoimmune disease are discussed.

Involvement of CCR5 in the passage of Th1-type cells across the blood-retina barrier in experimental autoimmune uveitis

Although the recruitment of T helper cell type 1 (Th1)/Th2 cells into peripheral tissues is essential for inflammation and the host response to infection, the traffic signals that enable the distinct positioning of Th1/Th2 cells are unclear. We have determined the role of CC chemokine receptor 5 (CCR5) in this using experimental autoimmune uveitis (EAU) as a model system. In EAU, Th1‐like cells are preferentially recruited into the retina across the blood‐retina barrier, partly as a result of expression of the adhesion molecules P‐selectin glycoprotein ligand 1 and lymphocyte function‐associated antigen‐1 on these cells. CD3+ T cells, infiltrating the retina, also expressed the chemokine receptor CCR5, and CCR5 ligands, macrophage‐inflammatory protein‐1α (MIP‐1α), MIP‐1β, and regulated on activation, normal T expressed and secreted (RANTES), were strongly expressed in the retina at peak EAU. Th1‐like cells, polarized in vitro, expressed high levels of CCR5. The trafficking of these CCR5+ cells was examined by tracking them after adoptive transfer in real time in vivo at an early disease stage using scanning laser ophthalmoscopy. Treatment of the cells with antibody against CCR5 prior to transfer resulted in a reduction in their infiltration into the retina. However, rolling velocity, rolling efficiency, and adherence of the cells to retinal endothelium were not reduced. CCR5 is clearly important for Th1 cell recruitment, and this study demonstrates for the first time in vivo that CCR5 may act at the level of transendothelial migration rather than at the earlier stage of rolling on the endothelium.

Differentiation to the CCR2+ Inflammatory Phenotype In Vivo Is a Constitutive, Time-Limited Property of Blood Monocytes and Is Independent of Local Inflammatory Mediators

It is proposed that CCR2+ monocytes are specifically recruited to inflammatory sites, whereas CCR2− monocytes are recruited to normal tissue to become resident macrophages. Whether these subsets represent separate lineages, how differential trafficking is regulated and whether monocytes undergo further differentiation is uncertain. Using a mouse model of autoimmune uveoretinitis we examined monocyte trafficking to the inflamed retina in vivo. We show that bone marrow-derived CD11b+ F4/80− monocytes require 24 to 48 h within the circulation and lymphoid system before acquiring the CCR2+ phenotype and trafficking to the inflamed retina is enabled. This phenotype, and the capacity to traffic were lost by 72 h. Monocyte CCR2 expression followed a similar time course in normal mice indicating that differentiation to an inflammatory phenotype is a constitutive, time-limited property, independent of local inflammatory mediators. Phenotypic analysis of adoptively transferred cells indicated that circulating inflammatory monocytes also differentiate into CD11c+ and B220+ dendritic cells and F4/80+ tissue macrophages in vivo. Our data supports the hypothesis of continuous extravasation and progressive differentiation over time of inflammatory monocytes in the circulation rather than replication within the actively inflamed tissue, and supports the concept of myeloid dendritic cell differentiation from trafficking monocytes under physiological conditions in vivo.

Experimental autoimmune uveoretinitis : a model system for immunointervention : a review

Experimental autoimmune uveoretinitis (EAU) is a useful model of human posterior uveitis and as such, permits the analysis of strategies for immuno-intervention. Modulation of the autoimmune response may be attempted at the stages of induction of EAU, during homing of autoreactive lymphocytes to the target organ, the retina, or during the effector stage of the disease. This paper presents a brief overview of current immuno-therapeutic modalities and assesses the usefulness for extrapolation to human disease.